Synthesis gas (hereinafter referred to as syngas) is a mixture of hydrogen (H2) and carbon monoxide (CO). Syngas can be produced, in principle, from virtually any feedstock material containing carbon. Carbonaceous materials commonly include fossil resources such as natural gas, petroleum, coal, and lignite. Renewable resources such as lignocellulosic biomass and various carbon-rich waste materials can also be used to produce syngas. It is preferable to utilize a renewable resource to produce syngas because of the rising economic, environmental, and social costs associated with fossil resources.
There exist a variety of conversion technologies to turn these various feedstocks into syngas. Conversion approaches can utilize a combination of one or more steps comprising gasification, pyrolysis, steam reforming, and/or partial oxidation of a carbon-containing feedstock.
Syngas is a platform intermediate in the chemical and biorefining industries and has a vast number of uses. Syngas can be converted into alkanes, olefins, oxygenates, and alcohols. These chemicals can be blended into, or used directly as, diesel fuel, gasoline, and other liquid fuels. Syngas can also be directly combusted to produce heat and power.
Since the 1920s it has been known that mixtures of methanol and other alcohols can be obtained by reacting syngas over certain catalysts (Forzatti et al., Cat. Rev.—Sci. and Eng. 33(1-2), 109-168, 1991). Fischer and Tropsch observed around the same time that hydrocarbon-synthesis catalysts produced linear alcohols as byproducts (Fischer and Tropsch, Brennst.-Chem. 7:97, 1926).
Technology developers for these catalysts have included Dow Chemical/Union Carbide and Institut Francais du Petrole. Dow Chemical and Union Carbide jointly developed a sulfided mixed-alcohol catalyst based on molybdenum, MoS2 (Phillips et al., National Renewable Energy Laboratory TP-510-41168, April 2007).
U.S. Pat. No. 4,752,623 (Stevens and Conway), originally assigned to Dow Chemical, discloses a catalyst for producing mixed alcohols from syngas, wherein the catalyst contains either molybdenum or tungsten, in addition to either cobalt or nickel, both components being in sulfided form. Stevens and Conway emphasize that it is not necessary for their invention that any particular stoichiometric metal sulfide be present. Sulfided cobalt is often assigned to CoS in the literature. Further, Stevens and Conway state that no advantage is realized by the presence of sulfur in the feed.
Another development at Dow Chemical, related to a similar catalyst preparation by Dianis, involves addition of aqueous cobalt acetate to ammonium molybdate in 30% acetic acid solution. Both the Dianis as well as the Stevens/Conway approaches employ decomposition under N2 at 500° C. Dianis mentions the presence of a peak in the powder X-ray diffraction pattern that is tentatively assigned to CoS2, but there is no discussion as to whether the presence of CoS2 is favorable or unfavorable (Dianis, Applied Catalysis 39, 99-121, 1987).
More recently, Iranmahboob and Hill (Catalysis Letters 78, 49-55, 2002) discussed similar catalysts for synthesis of higher alcohols. Iranmahboob and Hill found Co3S4 present in their better catalysts and Co9S8 present in their inferior catalysts. They hypothesized that, in their system, H2S evolution results in transformation of more-active Co3S4 into less-active Co9S8.
The existing art provides little, if any, information concerning chemical or physical characteristics that tend to correlate with the performance of cobalt-molybdenum-sulfide alcohol-synthesis catalysts, including Co—Mo—S, and similar catalyst systems comprising Ni and/or W. Particularly absent is information relating to preferred amounts of sulfur, on a stoichiometric basis, relative to other major components present. Also particularly absent is information relating to the preferred nature of the physical and/or chemical bonds or associations among at least Co, Mo, and S.
In light of these shortcomings in the art, what is needed is a novel and non-obvious discovery that reveals and distinctly teaches improved catalyst compositions in a manner that enables a person skilled in the art to make and use the catalyst compositions. Especially needed are preferred methods of making these catalyst compositions, and preferred methods of using these catalyst compositions, to convert syngas into alcohols. An especially preferred alcohol is ethanol, which can replace gasoline and other liquid fuels, at least in part, today.